When acclaimed, up-and-coming orchestral indie rock band San Fermin takes the stage at Summerfest on opening day, its Milwaukee-born frontwoman, Rae Cassidy, won't be there.

Cassidy announced on her Facebook page last week that she's left the New York band to focus on her solo career.

The news, and particularly the timing, is sudden and surprising. Cassidy's departure is just a few months before San Fermin will play for one of its biggest audiences to date at her hometown festival. And the band continues to increase in popularity--it played several gigs Austin, Texas during South by Southwest, and is booked for Lollapalooza--and Cassidy's charismatic presence was one of the best elements of San Fermin's set.

"I need to be fulfilling a lot of different corners of my soul that seemed to be drying up," Cassidy explained to the Journal Sentinel. "Not that the band wasn't a really awesome outlet. Bit I'm a writer and a creator and I produce. I was only getting to explore one dimension of my creative heart, and I can only do that for so long."

Asked why she didn't stick it out until at least Summerfest, Cassidy explained, "I can't be like, 'Let me ride it out for the fun stuff then up and ditch.' They need time to figure out what they're going to do and need time to rehearse a new person. They are going to record the next album and that person will be in it for a while. I can't commit to two or three more years."

Cassidy had already worked on some material for San Fermin's follow-up, set to be recorded this summer, and recorded some demos. Ellis Ludwig-Leone, the band's founder and primary songwriter, wasn't sure if Cassidy's work will end up on the album.

She was brought on as a replacement for Holly Laessig and Jess Wolfe, who were hired to sing on "San Fermin" in fall 2011. They played a couple shows in New York that led to a record deal, but Laessig and Wolfe were working on their own band Lucius and couldn't commit to San Fermin's touring plans--leading Ludwig-Leone on a search that led him to Cassidy.

"She's been a great, great friend and will continue to be," Ludwig-Leone said. "She's been a wonderful musician to work with and has done so much for the band. When we started with her we were playing for 40 people. Having grown it from there to where it is now has been a wonderful experience."

San Fermin is currently rehearsing with a new female vocalist in preparation for a European tour that starts April 15; Ludwig-Leone said he planned to reveal her identity before then. And Cassidy has moved from New York back home to Milwaukee, where she's writing songs and lining up collaborators for a solo album.

"I learned that chasing down my career, I need to enjoy everything. I don't want to kill myself over this," Cassidy said. "That's why I'm home. I could do this in New York, but I really want to enjoy every second and be home with my family."